'Pollution' is believed to be behind the deaths of a large number of fish in the River Kent in Kendal.
The Environment Agency is investigating what it describes as a 'major incident' after the fish were found dead last week.
A spokesperson said: “We responded to reports of a serious fish kill on the River Kent in Kendal.
"While the investigation is ongoing we would be unable to comment further. However we believe we have found the cause and further pollution has stopped.
“The Environment Agency takes pollution incidents very seriously. Anyone who spots pollution in the environment should report it to our free 24-hour hotline: 0800 80 70 60."
At this stage, it is not clear exactly what kind of pollution caused the deaths.
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Some local anglers think it will also seriously affect Brown Trout and stop Salmon swimming up the river to spawn.
Denis Noden, Kent Angling Association said: "I got down here to find thousands of small fish dead and the water was black.
"It was heartbreaking, we look after this river, we use it for angling, we support the fish that use it...make sure that everything's tiptop with the water itself.
"We make sure it's not being abused and then to find all this washed away basically in one incident."
The environment agency have told us what happened here is being treated as a category one incident where the environmental impact is major.
Denis continued: "We get Lamprey, Sticklebacks, Minnows, young Salmon, Sea Trout all come up, so there are lots and lots of different types of fish that rely on this bit of river and they've all been killed."
Anglers say they're worried its effects on the river could take years to overcome.